When responding to a recent post, I attempted to upload a style file. I got this message:
The file type (.stylx) is not supported. Valid file types are: jpg, gif, png, pdf, xlsx, docx, pptx, txt, log, csv, mp3, mp4, avi, har, Mxd, Mpk, Lpk, 3dd, Sxd, Lyr, gpk, aprx, ppkx, mpkx, lpkx, mmpk, mspk, lyrx, gpkx, gcpk, pagx, rpk, .esriTasks, zip
It's clear that lots of Esri-specific file types are allowed here, but not style files! It's not a big deal to zip up my file and add it that way, but I was curious that so many custom filetypes are in the "OK" list, but this one wasn't.
I haven't looked extensively at all the other filetypes, but I would additionally suggest that any file someone might export and share from Pro ought to be possible to upload to the Community.